1. Field of the Invention
The present invention relates to a toner and a production process for the same, the toner used for developing an electrostatic image formed on a photosensitive member provided in an image forming apparatus such as copiers and printers. More particularly, the invention relates to a toner which includes core particles containing at least a resin and a colorant and coated with a resin coating layer, and to a process for producing the toner.
2. Description of the Related Art
The image forming apparatuses such as copiers and printers have conventionally used toners for developing an electrostatic image formed on the photosensitive member.
A milling process is widely used for producing such a toner. The process includes the steps of: admixing additives including a colorant, a wax and the like to a resin; melting the mixture by heating and kneading the molten mixture; cooling the kneaded product; and milling the product into toner particles of a predetermined particle size.
Unfortunately, in a case where the toner is produced by the milling process, the produced toner has various problems such as great variations of particle size, poor productivity, and high production costs. In a case where a toner of a small particle size is produced, in particular, a yield is seriously decreased.
More recently, therefore, an emulsion polymerization/flocculation method has been proposed as a toner production method which allows for arbitrary control of the toner particle configuration or of toner particle size distribution.
Where the toner is produced by the aforesaid emulsion polymerization/flocculation method, the following procedure may be taken. A dispersion of resin microparticles is prepared by emulsion polymerization. On the other hand, a dispersion of colorant microparticles is prepared while, as required, a wax dispersion to be used as a wax, or the like, is prepared. These dispersions are blended together and stirred while a suitable flocculating agent such as an inorganic metal salt is added to the dispersion mixture so as to allow the above resin microparticles, colorant microparticles and such to flocculate together. Subsequently, the resultant flocculate is fusion-bonded by heating and thus is produced the toner.
Where the toner is produced in this manner, however, the colorant or the wax used as the wax is exposed to toner surfaces so that the toner is lowered in stability to environment. Hence, the toner is varied in electric charge due to the environmental changes, so that formed images may suffer density variations or fogging. Furthermore, in the case of color image formation, formed images may suffer color tone changes.
More recently, there has been proposed a capsule toner including a core material containing at least a binder resin and a colorant and encapsulated in resin capsules (Japanese Unexamined Patent Publication No. 5-297622).
Unfortunately, however, the capsule toner has the following problem. It is difficult to control the particle size distribution of the toner because the capsule toner is produced by interfacial polymerization of oil-in-water emulsion. As a consequence, the particle size distribution of the capsule toner is too broad.
Furthermore, there has been proposed an alternative toner including at least a binder resin and an encapsulated colorant (Japanese Unexamined Patent Publication No. 11-7163).
However, this toner has a drawback that it is impossible to fully prevent the colorant from being exposed to the surface of toner particles.
There is also proposed a toner wherein resin particles are made to adhere or fix to flocculated particles containing at least primary polymer particles and a colorant, the above primary polymer particles admixed with a wax (Japanese Unexamined Patent Publication No. 2002-82487).
However, the toner has the following problems. In the production of the toner, the primary polymer particles and the colorant particles are flocculated at a temperature lower than a glass transition point Tg of the primary polymer particles and then the resultant flocculate is fusion-bonded. Therefore, when the resin microparticles are made to adhere or fixed to the flocculated particles for forming the toner, the flocculated particles are mixed with the resin microparticles so that it becomes impossible to control the characteristics of the toner which include fixing performance, thermostability/storability, anti-stress performance and the like.